https://leetcode.cn/problems/account-balance-after-rounded-purchase/?envType=daily-question&envId=2024-06-12
class Solution {
public:
    int accountBalanceAfterPurchase(int purchaseAmount) {
        int x = purchaseAmount %10;
        int ret = 100 - purchaseAmount/10*10;
        if(x>4){
            ret-=10;
        }
        return ret;
    }
};